Add keyword parameters to functions

[clinssen]

Spin-off from #946.

Implement a more Python-like syntax for function calls, where the attribute names can explicitly listed. For instance, if we define

function foo(weight real, delay ms) real:
    emit_spike(weight, delay)

then instead of

foo(w, 1 ms)

we could write

foo(weight=w, delay=1 ms)

or

foo(w, delay=1 ms)

but not

foo(weight=w, 1 ms)

because keyword-value pairs should always come after index-based parameters, following Python.